The cylinder arbor (the part the cylinder rotates on) seems to be either missing or broken off.

The gun is a general type of European revolver of the c. 1870-1900 era. I am pretty sure it was made in Belgium and if you look closely at the back of the cylinder you might find an oval with the letters E L G, the Liege proofmark.

Those guns are fairly common, even in the U.S. and are of little value, perhaps $200 tops if in excellent condition and working properly (which they rarely are).